Preferential inhibition of oω-conotoxin-sensitive presynaptic Ca ²⁺ channels by adenosine autoreceptors

ADENOSINE is a potent modulator of transmitter release at a variety of synapses. The adenosine Al receptor is assumed to reside in presynaptic terminals and to function as a negative autoreceptor¹. How adenosine reduces transmitter release is uncertain²; it may reduce the calcium influx during nerve terminal depolarization by either activating K⁺ currents^3,4 or inhibiting Ca²⁺ currents^5,6, although other mechanisms have been proposed^7-9. We have directly measured intracellular Ca²⁺ concentrations of giant presynaptic terminals in the chick ciliary ganglion. We report here that adenosine inhibited the nerve-evoked Ca²⁺ influx in the termi-nal by activating Al receptors. Reduced Ca²⁺ influx was due largely to inhibition of ω-conotoxin GVIA-sensitive Ca^{2 +} channels in the presynaptic terminal.